From: "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" <longpeng2@huawei.com>
To: "Marc-André Lureau" <marcandre.lureau@gmail.com>,
"Daniel P. Berrangé" <berrange@redhat.com>
Cc: chenjiashang@huawei.com, Markus Armbruster <armbru@redhat.com>,
Peter Xu <peterx@redhat.com>, QEMU <qemu-devel@nongnu.org>,
"Gonglei \(Arei\)" <arei.gonglei@huawei.com>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: A bug of Monitor Chardev ?
Date: Tue, 25 May 2021 14:53:28 +0800 [thread overview]
Message-ID: <6b0a3f5a-f289-6eed-9a11-a378eeb45e32@huawei.com> (raw)
In-Reply-To: <CAJ+F1CL_fTN8W2McRA-XXmY6HTy47GZdHz8aB4wxee_7hnyL2g@mail.gmail.com>
Hi Marc,
在 2021/5/22 0:59, Marc-André Lureau 写道:
> Hi
>
> On Fri, May 21, 2021 at 8:56 PM Daniel P. Berrangé <berrange@redhat.com
> <mailto:berrange@redhat.com>> wrote:
>
> On Fri, May 21, 2021 at 05:33:46PM +0100, Daniel P. Berrangé wrote:
> > On Fri, May 21, 2021 at 10:43:36AM -0400, Peter Xu wrote:
> > >
> > > I think the original problem was that if qemu_chr_fe_set_handlers() is
> called
> > > in main thread, it can start to race somehow within execution of the
> function
> > > qemu_chr_fe_set_handlers() right after we switch context at:
> > >
> > > qemu_chr_be_update_read_handlers(s, context);
> > >
> > > Then the rest code in qemu_chr_fe_set_handlers() will continue to run in
> main
> > > thread for sure, but the should be running with the new iothread
> context, which
> > > introduce a race condition.
> > >
> > > Running qemu_chr_be_update_read_handlers() in BH resolves that because
> then all
> > > things run in the monitor iothread only and natually serialized.
> >
> > The first message in this thread, however, claims that it is *not*
> > in fact serialized, when using the BH.
> >
> > > So the new comment looks indeed not fully right, as the chr device should be
> > > indeed within main thread context before qemu_chr_fe_set_handlers(),
> it's just
> > > that the race may start right away if without BH when context switch happens
> > > for the chr.
> >
> > It sounds like both the comment and the code are potentially wrong.
>
>
> I feel like our root cause problem that the original code was trying to
> workaround, is that the chardev is "active" from the very moment it is
> created, regardless of whether the frontend is ready to use it.
>
> IIUC, in this case the socket chardev is already listen()ing and
> accept()ing incoming clients off the network, before the monitor
> has finished configuring its hooks into the chardev. This means
> that the initial listen()/accept() I/O watches are using the
> default GMainContext, and the monitor *has* to remove them and
> put in new watches on the thread private GMainContext.
>
> To eliminate any risk of races, we need to make it possible for the
> monitor to configure the GMainContext on the chardevs *before* any
> I/O watches are configured.
>
> This in turn suggests that we need to split the chardev initialization
> into two phases. First we have the basic chardev creation, with object
> creation, option parsing/sanity checking, socket creation, and then
> second we have the actual activation where the I/O watches are added.
>
> IOW, qemu_chr_new() is the former and gets run from generic code in
> the main() method, or in QMP chardev_add. A new 'qemu_chr_activate'
> method would be called by whatever frontend is using the chardev,
> after registering a custom GMainContext.
>
> This would involve updating every single existing user of chardevs
> to add a call to qemu_chr_activate, but that's worth it to eliminate
> the race by design, rather than workaround it.
>
>
>
> What about my earlier suggestion to add a new "qemu_chr_be_disable_handlers()"
> (until update_read_handlers is called again to enable them and the set a
> different context)?
>
In this case, the BH calls the update_read_handlers, so the new added
"qemu_chr_be_disable_handlers" will be called in the monitor iothread BH ? If
so, I'm not sure whether it is safe enough, because the Chardev may still be
accessed in parallel by main loop and iothread for a while.
How about call "qemu_chr_be_disable_handlers" before set the
monitor_qmp_setup_handlers_bh ?
I think Daniel's soluation is perfect, but it's beyond my ability, I'm not
expert in Chardev/QMP, it's difficult to guarantee no other bugs will be
introduced, so we prefer to take the simplest and safest way to fix the bug in
our production.
>
> --
> Marc-André Lureau
next prev parent reply other threads:[~2021-05-25 6:55 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-17 6:56 A bug of Monitor Chardev ? Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
2021-05-19 16:17 ` Marc-André Lureau
2021-05-19 16:40 ` Daniel P. Berrangé
2021-05-21 7:25 ` Markus Armbruster
2021-05-21 14:43 ` Peter Xu
2021-05-21 16:33 ` Daniel P. Berrangé
2021-05-21 16:56 ` Daniel P. Berrangé
2021-05-21 16:59 ` Marc-André Lureau
2021-05-21 17:07 ` Daniel P. Berrangé
2021-05-21 17:14 ` Peter Xu
2021-05-25 6:53 ` Longpeng (Mike, Cloud Infrastructure Service Product Dept.) [this message]
2021-05-21 17:09 ` Peter Xu
2021-05-21 17:15 ` Daniel P. Berrangé
2021-06-08 14:07 ` Markus Armbruster
2021-06-08 15:37 ` Daniel P. Berrangé
2021-06-09 0:20 ` Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
2021-06-09 10:13 ` Marc-André Lureau
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